Tumblr Architecture - 15 Billion Page Views A Month

Confirmation that MySQL scales just fine with sharding. More details on their sharding implementation here.With all the buzz around NoSQL I think people are underestimating good old SQL databases like MySQL or PostgreSQL for example.

Confirmation that Redis is just great.

Assigning users to Cells helps handle the combinatorial explosion of users x followers x posts.To draw an analogy with Ethernet networking, this reminds me of how you can segment a LAN using network switches to reduce bandwidth usage and congestion (as some of the traffic will stay within each cell / segment).